1. Field of the Invention
The present invention relates to color cathode ray tubes (CRTs), and more particularly, to a tension mask with a color selection function and a tension mask and frame assembly for a color CRT.
2. Description of the Related Art
In color CRTs, three electron beams emitted from an electron gun land on a phosphor screen through apertures in a shadow mask with a color selection function to excite red, green and blue phosphor lines in the phosphor screen formed on the inner surface of a panel. The panel of a conventional color CRT, which forms an image as mentioned above, is designed with a predetermined inner curvature taking into account a deflection trajectory of electron beams, which have been deflected by a deflection yoke after being emitted from the electron gun. The shadow mask thereof is also designed with a curvature corresponding to that of the panel.
In the operation of a CRT, the mask manufactured to have a curvature in the same order as that of the inner surface of the faceplate is heated as kinetic energy of electron beams emitted from the electron gun is converted to thermal energy, to thereby distort the mask into the shape of a dome resulting from thermal expansion. which is referred to as a xe2x80x9cdoming phenomenonxe2x80x9d. The doming phenomenon displaces positions of the apertures formed on the mask, so that the landing positions of the electron beams are shifted. As a result, undesired phosphor lines are excited, so that the color purity of a display image is deteriorated.
The mask described above is made of steel foil having a thickness of 0.1-0.25 mm. A plurality of apertures are formed in the steel foil via etching, and then the steel foil is molded to have a predetermined curvature. If the curvature of the mask is less than a predetermined level, the mask is readily subjected to a permanent thermal distortion during manufacture of CRT. Accordingly, the mask cannot perform a normal color selection function due to its structural weakness.
Also, the mask having the above-mentioned configuration has limitations in the manufacture of flat CRTs, while the need for flat CRTs is increasing.
In order to meet the need for the flat CRTs as well as to prevent the doming of the mask, U.S. Pat. No. 3,638,063 has suggested an aperture grill type mask. As shown in FIG. 1, a mask 10 including a plurality of strips 12, grid elements, which are disposed at predetermined intervals, is supported by a frame 11 under one directional tension. The mask 10 having the grid structure is designed such that thermal strain which occurs during operation of a CRT, can be opposed by the tension applied by the mask-and-frame assembly, thereby preventing doming of the mask. However, the mask 10 with the strips 12 is made of steel foil that is 0.1 mm thick and is only supported by the frame 11 at two edges thereof, without any interconnection between adjacent strips, and thus the individual strips 12 easily vibrate when subjected to a small impact, thereby causing a howling phenomenon to arise in a display image. The tension applied to the strips 12 is proportional to the thickness of a single strip. Thus, in order to make the structural intensity withstand the thermal expansion during CRT operation, the weight of the frame 11 is inevitably increased.
To account for this problem, U.S. Pat. No. 4,942,332 discloses a mask illustrated in FIGS. 2 and 3. As shown in FIGS. 2 and 3, a mask 20 includes a series of parallel strips 22 disposed at predetermined intervals to define slits 21, and a plurality of tie bars interconnecting adjacent strips 22. Also, the longer sides of the mask are fixed to a support member (not shown).
In the mask 20, the tie bars 23 interconnecting the adjacent strips 22 can reduce howling of a display image, resulting from vibrations of the mask by an external impact, but not contributing to a reduction in the Poisson contraction. In particular, when a tension is applied in the vertical direction of the mask within the elastic limit of the material of the mask, the mask 20 is stretched in the longitudinal direction, but contracted in the transverse direction. Due to the transverse contraction, the outermost slits of the mask 20 are displaced. Furthermore, as thermal expansion raises the mask 20 during operation of a CRT, the shorter sides of the mask 20 expand outward.
The disclosure also defines a vertical-to-horizontal pitch ratio of the slits (PV/PH) to be greater than 16 such that the transverse displacement of the slits at the side of the usable picture region of the mask.
The instant inventors has carried out the following numerical analysis during research into the amount of displacement of slits at the edge of a mask with respect to vertical-to-horizontal pitch ratio (PV/PH) of slits. In particular, the specification of the mask and experimental conditions therefor were: the width of the mask (W) was 298.4 mm; the height of the mask (H) was 312 mm; the horizontal pitch (PH) of slits was 0.8 mm; the width of a single strip (W1) was 0.6 mm; the thickness of the mask (t) was 0.1 mm; Young""s Modulus (E) was 2.1xc3x9710xe2x88x926 kgf/mm2; the thermal expansion coefficient (xcex1) was 13xc3x97106/xc2x0 C.; Poisson""s ratio was 0.27; and the material used was aluminum killed (AK) steel.
In the mask having the above specifications. it was assumed that the area of the usable picture region was 387.4xc3x97288 mm, and that the total number of strips was 484. To keep the tension constant when the temperature is raised to 100xc2x0 C., the amount of initial tension applied to the strips of the mask, expressed by xcex5=xcex1xcex94T should be equal to 0.0013 (=13xc3x9710xe2x88x926/100), wherein the tension applied to a single strip, expressed by ƒstrip="Egr"xc2x7xcex5xc2x7 (area of a single strip), is equal to 1.638 kgf. Also, the amount of transverse contraction of a single strip, expressed by xcex94W1 =xcexdxc2x7xcex5xc2x7W1, is equal to 0.211 xcexcm. Here, since the total number of strips is 484, after application of the tension, a maximum transverse contraction of the useful picture region (xcex94W) becomes 102 xcexcm. As a result the edge of the useful picture region is shifted outward by 52 xcexcm.
FIG. 4 comparatively illustrates the amount of transverse displacement of slits in the disclosure of U.S. Pat. No. 4,942,332 and the numerical analysis by the instant inventors, with respect to the vertical-to-horizontal pitch ratio (PV/PH) of the slits. As shown in FIG. 4, according to the disclosure, the transverse displacement of the slits was 64 xcexcm for a PV/PH of 5; 8 xcexcm for a PV/PH of 15; and 15 xcexcm for a PV/PH of 30. Meanwhile, the numerical analysis performed under the same conditions as those of the disclosure showed a transverse displacement of 50 xcexcm for a PV/PH of 5 and 32.2 xcexcm for a PV/PH of 30.
As can be noted from the above result, a decreasing tendency of the displacement with a PV/PH increase appears both in the disclosure and the numerical analysis result. However, a decrease in the Poisson contraction at the edge of the useful picture region of the mask appears to be exaggerated in the disclosure, in contrast to the numerical analysis.
In detail, according to the numerical analysis by the instant inventors, at a PV/PH of 30, the outermost slits of the mask contract by 33.7 xcexcm from the initial positions upon the application of tension, while the outermost slits expand by 28.1 xcexcm from the initial positions when the temperature of the mask increases to 80xc2x0 C. during operation of a CRT. Accordingly, the total amount of the transverse displacement of the outmost slits, arising from the Poisson contraction and the thermal expansion, becomes 61.8 xcexcm.
As described above, the amount of Poisson Contraction can be somewhat reduced by increasing the PV/PH ratio of slits as indicated in the disclosure. However, unlike the prediction in the prior art, it is likely that increasing the PV/PH ratio is insufficient for reducing the absolute transverse contraction of the mask. Furthermore. if the PV/PH ratio is too high, due to the strips being loosely coupled by widely spaced tie bars, strips are easily subjected to vibration, resulting in an increasing concern about image quality deterioration.
An object of the present invention is to provide a tension mask and a tension mask and frame assembly for color cathode ray tubes (CRTs), which render a biaxial tension to a mask, one in the longitudinal direction (Y-axis direction) of strips and the other in the direction (X-axis direction) of tie bars interconnecting adjacent strips, so that the amount of displacement of slits at the edges of the mask can be reduced.
Another object of the present invention is to provide a tension mask and a tension mask and frame assembly for color CRTs, capable of reducing the amount of Poisson contraction in the longitudinal direction of the mask to a minimum level.
To achieve the first object of the present invention, there is provided a tension mask for a color cathode ray tube (CRT), having a partial pin cushion shape, comprising an aperture portion including a series of strips disposed at predetermined intervals to define slits, and tie bars interconnecting adjacent strips, wherein strips near the side of the aperture portion have a predetermined partial pin cushion curvature; and a non-aperture portion having side members disposed parallel to the strips on both sides of the aperture portion, the side members also having a predetermined partial pin cushion curvature, wherein upon a tension is applied to the tension mask in the vertical direction of the strips and the side members, a tension in the horizontal direction of the strips is also applied to the tension mask.
Preferably, assuming that the maximum curvature of the tension mask is xcex4, and the height of the tension mask is H and the width of the tension mask W. the relationship 0.00027W/2 less than xcex4 less than 0.01 H is satisfied. Also, assuming that the vertical pitch of the slits is PV and the horizontal pitch of the slits is PH, a vertical-to-horizontal pitch ratio PV/PH is greater than or equal to 2.
To achieve the second object of the present invention, there is provided a tension mask and frame assembly for a color cathode ray tube (CRT), comprising: a tension mask including an aperture portion having a series of strips disposed at predetermined intervals to define slits, and tie bars interconnecting adjacent strips, and a non-aperture portion having side members disposed parallel to the strips on both sides of the aperture portion, wherein strips near the side of the aperture portions and the side members have a predetermined partial pin cushion curvature, and a frame including a pair of support members disposed parallel to each other, for supporting the tension mask under tension acting in the vertical direction of the strips, such that a tension acts on the horizontal direction of the tension mask through the tie bars, due to the curvature of the tension mask, and a pair of elastic members fixed to both ends of the support members for supporting the support members.